Attention-Enhanced Caption Decoder  
The left sub-figure in Figure REF  shows the widely-used attention-enhanced LSTM decoder [1]}, [2]}.
For each decoding step \(t\) , the decoder takes the word embedding of current input word \(y_{t-1}\) , concatenated with the averaged visual features \(\bar{v}=\frac{1}{k} \sum _{i=1}^{k} v_{i}\)  as input to the LSTM:
\(\footnotesize h_{t}=\text{LSTM}\left(h_{t-1}, [W_e y_{t-1};\bar{v}]\right) ,\) 
